This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The in situ stimulation of dissimilatory iron-reducing bacteria (DIRB) has received considerable attention as an effective means of environmental remediation. DIRB couple the oxidation of organic carbon to the reduction of Fe(III)(hydr)oxides resulting in the generation of Fe(II). Iron(II) can then precipitate with contaminants in solution or reduce soluble redox-active metals to their insoluble reduced states. Investigations of microbial Fe(III) reduction have consistently revealed either a minimal or diminished reductive capacity of Fe(III) (hydr)oxides. Based on preliminary investigations, we hypothesize that structural and compositional characteristics inherent to the Fe(III) phases are responsible for their compromised reactivity. In particular, substitution of Fe(III) by foreign ions will alter the solubility and reactivity of Fe phases resulting in a diminished Fe(III) reduction capacity and altered biomineralization pathway relative to pure Fe(III) phases. Thus, the objective of this research is to define the rates of Fe(III) reduction and secondary mineralization reactions of four common Fe(III) (hydr)oxide phases (ferrihydrite, lepidocrocite, goethite, and hematite) containing a range of co-precipitated ions (Al, Mn, Ni, Si) in the presence of DIRB (Shewanella putrefaciens CN32) and an abiotic reductant (ferrous Fe). A critical component of this research is to define the fate or Fe(II) and the secondary phases formed following reduction of Fe(III) as a function of the initial Fe (hydr)oxide structure and the composition and concentration of substituted ions. Considering that contaminant transport is a function of the reducing capacity of Fe(III)(hydr)oxide substrates, the fate (reactivity) of generated Fe(II), and secondary phase precipitation, this research will enhance our predictive capability of the potential success of Fe(II)-based remediation approaches.